Ferritic stainless steel is inferior compared with austenitic stainless steel in workability, toughness, and high temperature strength, but does not contain a large amount of Ni, so is inexpensive and, further, is small in heat expansion, so is used for materials for exhaust system parts of automobiles etc. In general, SUH409L, SUS429, SUS430LX, SUS436J1L, SUS432, SUS444, and other steel types are used as ferritic stainless steel suitable for these applications.
These materials are used shaped into pipes etc. Further, for the flange materials for connecting parts worked into these pipes etc. with each other (automobile flange materials), ordinary steel, even though inferior in corrosion resistance, is mainly being used. In recent years, the most inexpensive ferritic stainless steel SUH409L has also been used.
However, due to the need to lighten the automobile body weight and to extend service life etc., materials which are excellent in corrosion resistance are also being demanded for automobile flange materials. Ferritic stainless steel of SUH409L or more is being used. Further, in the case of use for exhaust systems, there is also the effect that the higher the strength at high temperatures, the thinner the sheet thickness can be designed, so ferritic stainless steel is advantageous over ordinary steel.
For use for automobile flange materials, in some cases, thickness 3 mm or less thin cold rolled steel sheet is used while improving the rigidity by bending etc., but in most cases thickness 5 mm or more thick hot rolled steel sheet is used as it is by just stamping.
However, thickness 5 mm or more hot rolled steel sheet of ferritic stainless steel is low in toughness, so is difficult to manufacture. In production of thickness 5 mm or more hot rolled steel sheet of ferritic stainless steel, the sheet often breaks on the production line after hot rolling. Therefore, up to now, studies on improving toughness have mainly started from the production aspect.
PLT 1 discloses a method comprising causing a finishing temperature at the time of hot rolling to change in accordance with the alloy composition, coiling, then rapidly cooling. Both PLT 2 and PLT 3 shows methods of improvement of toughness for the purpose of improving the manufacturability of thick gauge hot rolled coil.
When working ferritic stainless steel as an automobile flange material, in most cases stamping is used for production. Therefore, ferritic stainless steel which is inferior in toughness is disadvantageous. In particular, in stamping work in the winter, cracking often occurs and production of parts is difficult. Therefore, ferritic stainless steel sheet which is excellent in toughness and therefore free from hindering production of parts even in the winter has been desired.